Such a pumped volume totalizer provides information that may be used for many purposes, including measuring and controlling the inflow to a sewage plant or measuring and reporting discharge of effluent. If the level in the vessel is measured by a continuous level measurement system, such as an ultrasonic level measuring device, then it is a simple matter to calculate the volume represented by changes in level as long as the shape of the vessel, and therefore the volume contained in the vessel at any particular liquid level is known. One simple technique is to measure a start level when a pump cycle begins as a pump control system turns on one or more pumps to pump liquid out of (or into) the vessel. When the level in the vessel has dropped (or risen) to a predetermined value, the pump control system turns the pumps off, at which point the level is again recorded, providing a stop level. The volume pumped is the difference between the calculated volume contained in the vessel at the start level and the calculated volume contained in the vessel at the stop level. This volume can then be made available for display or adding to a running total of volume pumped, or to control an external totalizing counter.
Such a system makes no allowance for any liquid flowing into or out of the vessel during the pump cycle other than through the pumps. The calculation assumes that any change in volume that occurs between the start and end of the pump cycle is entirely due to the action of the pumps. In a typical application, liquid may be flowing into (or out of) the vessel during the pump cycle; indeed, it will often be such a liquid flow which causes a change in level which initiates the pump cycle. Thus the actual amount pumped in such circumstances will be the amount calculated as above, plus the amount that has flowed into the vessel during the pump cycle.
In order to overcome this problem, it is known to measure the rate of change of level in the vessel that is occurring just prior to the start of the pump cycle. It is then assumed that the inflow during the pump cycle was continuous and at the same rate as the value recorded just prior to the start of the pump cycle. The calculated pumped volume is then adjusted by adding to it the assumed total volume that flowed into the vessel during the pump cycle.
Although widely used, the assumption upon which a system is based, i.e. that the rate of inflow remains constant during the pump cycle, is often unjustified. For example, in a system handling storm water, the rate of inflow may vary widely during the course of a storm in a manner such that the average rate of inflow during the time that the pumps were running differed widely from that existing when the pumps cycle started, leaving two substantial errors.